Does Episerver support contacts with non unique email addresses? As in can we share email address across Epi Contacts? Does it have any repurcussions w.r.t Orders?
W.r.t login, I'm assuming we'd revert to unique username instead of unique email.
As I recall the Email field on the Contact class does not have to be unique, meaning you can do as you wrote.
Only the ContactId and the UserId fields are unique fields. But when populating the UserId field, you need to use a unique UserId or UserName. (Hint: use the MapUserKey class to convert from a UserName string to a Contact's UserId).
A Commerce contact can change their email address and can have multiple accounts with the same email address but the user name can't be changed for an account. Similarly while registering a customer if you are assigning an email address as the username then you can not change this username.